**Core Concept**
The question tests the understanding of late complications of acetabular fractures, focusing on the potential long-term effects on the hip joint and surrounding structures. **Acetabular fractures** can lead to various complications due to the complex anatomy of the hip and the proximity of vital structures. Understanding the potential for **avascular necrosis** and **osteoarthritis** is crucial.

**Why the Correct Answer is Right**
The correct answer, **Secondary osteoarthritis of hip joint**, is a well-recognized late complication of acetabular fractures. This occurs due to the altered joint mechanics and potential damage to the articular surface during the fracture, leading to degenerative changes over time. The **hip joint** is particularly susceptible to these changes due to its weight-bearing role and the importance of precise articulation for normal function.

**Why Each Wrong Option is Incorrect**
**Option A:** Avascular necrosis of the head of the femur is indeed a potential complication of hip fractures or dislocations but is not the most direct late complication of an acetabular fracture itself.
**Option B:** Avascular necrosis of the iliac crest is not a commonly recognized complication of acetabular fractures.
**Option C:** Fixed deformity of the hip joint could be a complication of inadequate reduction or malunion but is not as specific to the late effects on the joint as osteoarthritis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **acetabular fractures** can have significant long-term consequences on hip function, with **secondary osteoarthritis** being a major concern due to its impact on patient quality of life and the potential need for future joint replacement surgery.

**Correct Answer:** D. Secondary osteoarthritis of hip joint